In a conventional weather resistance testing apparatus or fademeter, such as shown in FIG. 1 and FIG. 2, an arc-lmap housing 1 is hung from the top of a test vessel 2 in the center thereof, and a specimen rack 3 which rotates around the lamp housing 1 or shaft 3a driven by a driving means (not shown) is adapted to have a test specimen 4 mounted thereon. Carbon electrodes 5 and 5' at the center of the arc-lamp housing 1 are mounted on holders 6 and 6' which in turn are driven up and down by chains 7, gear mechanism 8 and a motor 9, and the discharge arc is produced between electrodes 5 and 5'. The light produced passes through a light filter 10 and irradiates the sample. An exhaust port 11 is provided at the upper part of the lamp housing to remove high-temperature gases and ashes that are produced during the discharge between the electrodes. The gas in the lamp housing is blown out to the open air by an exhaust blower 12. Cool air is introduced into the lamp housing by being drawn downwardly through two intake pipes 13 on the right and left of the lamp housing and into the bottom of the lamp housing from the lower ends of the introduction pipes.
Thus, the air in the lamp housing is forcibly exhausted. The whirling air creates a disturbed vortex which flows upwardly and into the exhaust port, as can be observed if tobacco smoke or the like is introduced into pipes 13. Therefore, the flame of the discharge arc tends to be oscillated by the disturbance in the air, causing the voltage and current of producing the discharge to become unstable. Furthermore, ashes produced by the discharge arc are carried by the disturbed air stream, scattered and adhered onto the light filter. The ashes adhered onto the filter interrupt the transmission of the light and the amount and intensity of the light energy reaching the specimen tends to be reduced, thereby reducing the effectness of the apparatus for its intended purpose.
The lamp also radiates heat together with light, and the temperature of air in the test vessel rises gradually. To keep the temperature constant, a valve adjusting device including a temperature sensing device 14 is provided in the testing vessel 2, and a change over valve 15 driven by the valve adjusting device is provided between exhaust duct 19 and intake duct 17 to change the air flow pattern to introduce external cool air into the vessel 2 to replace hot air to reduce the temperature. When the temperature in the vessel becomes higher than a set value, the temperature sensing device 14 causes the valve adjusting device to change valve 15 from the position shown by a dotted line in FIG. 2, in which air from vessel 2 is recirculated into duct 17, to the state shown by a solid line. With valve 15 in this position, external cool air is drawn by a blower 16 into duct 17 and blown into the testing chamber 2 after passing through moisture-adding vessel 18. The air introduced into the testing vessel cools the surfaces of the specimen 4 and is forced past the valve 15 and exhausted into open air through exhaust duct 19. The hot air in the vessel is thus replaced by external cool air. When the external air is introduced into the vessel, the pressure created by the blower will be a positive pressure of a few millimeters of water.
The pressure in the lamp housing 1 on the other hand, will be negative pressure due to the suctions being exerted by the exhaust blower 12. Therefore, there develops difference in pressure between the interior of the lamp housing and the testing vessel 2, and the air in the testing vessel 2 flows rapidly into the lamp housing 1 through a clearance between the lamp housing 1 and the light filter 10, as shown by the arrows in FIG. 2, which is a cause of air disturbance in the lamp housing, resulting in unstable discharge. To prevent the occurance of this condition, the filter 10 should have a very flat surface and the surface of the lamp housing to which the filter is attached should be so finished as to maintain close contact with the filter. In practice, however, the heat from the lamp causes bending and sagging of the parts, giving rise to the development of a clearance between them. Therefore, the discharge arc often becomes unstable with each movement of the changeover valve 15. Sudden replacement of air is also a cause of varying air temperature in the testing vessel. The air in the testing vessel usually has a temperature of about 40.degree.C., and even if the temperature of the external air is about 20.degree.C., the introduction of air having a temperature difference of 20.degree.C. causes temperature hunting. This causes a temperature variation, as measured by a blackboard thermometer, of about .+-.3.degree.C. around 63.degree.C. which is a rough indication of the surface temperature of the specimens.